The invention relates to a gear unit having a plurality of reduction stages, particularly for use in a helicopter, and intended for a relatively high output torque with a relatively low speed of rotation and a total transmission ratio between about 70 and 150, with at least one stage with non-parallel shafts.
Gear units of the above-described type, in which at least one stage has non-parallel shafts, for power transmission systems are usually gear units the construction of which already entails technical difficulties. When in addition they are used in aircraft such as helicopters, the further problem arises of keeping their weight and size as low as possible, perhaps at the expense of long life, provided that reliable operation can be ensured during the possibly shortened life. If the output shaft of the gear unit is the shaft which carries and drives the main rotor of the helicopter, the problems to be overcome are even considerably greater, because all the forces, accelerations and moments which during flying operations, during maneuveres, starting and landing are exerted by the main rotor have to be transmitted to the body of the helicopter through the gear unit in question. At the same time the propulsion power must also be transmitted. The gearbox is therefore acted upon and loaded not only by the driving torque but also by the considerable external forces. A sufficiently strong light-weight gearbox can be built, but its rigidity will leave much to be desired, particularly in respect of the achievement of good running behaviour of the gears contained in it. Gear units are thus often used in practice which by nature are self-correcting within certain limits, such as for example self-centering planetary gear units. However, every helicopter contains at least one transmission between non-parallel shafts, frequently at right angles with intersecting axes, although this is not necessarily the case. Only few non-parallel transmission types are known which reasonably well meet the requirements listed above, although in practice they frequently constitute the weakest link in the entire gear train. In most known transmissions, in which the power source is a gas turbine with a power output shaft with a speed of about 20,000 revolutions per minute or more, and in which the main rotor usually has a speed of about 200 revolutions per minute, the required reduction ratio is usually achieved by means of at least three or four stages. Only in the case of helicopters having a single gas turbine, such as small civil helicopters, is it possible to manage with a single transmission with non-parallel shafts in the transmission chain. Most helicopters have two gas turbines, which are frequently placed symmetrically on opposite sides of the vertical rotor shaft and themselves have an approximately horizontal axis and output shaft. Therefore, in this configuration, which is by far the one most used, two transmissions between non-parallel shafts are even required for the power flow of each gas turbine. In view of the great differences in the speeds between the input and output transmission shafts, a total transmission ratio between about 70 and 150 being necessary, the last reduction stages are for understandable reasons disposed directly under the wings because of their large diametral dimensions. The right-angle transmission is placed approximately in the middle of the transmission, for transmitting moderate torque and speed. For this most difficult stage, use is generally made of a bevel gear transmission. The following list broadly shows their most important characteristics:
Type 1: Bevel gears having straight or oblique teeth can be ground. Their loadability is lower, the required accuracy of adjustment is higher, and their sensitivity to elastic bending of the gears themselves and of the teeth alone is greater than in the case of type 2. The transmission ratio is limited to about 5. PA0 Type 2: In the case of spiral-toothed bevel gears the loadability is greater, the accuracy of adjustment required is lower, although adjustment is still difficult, and sensitivity to elastic bending of the gears themselves and of the teeth alone is lower than in the case of type 1. The transmission ratio is limited to about 5. Considerable tooth flank friction occurs.
a. Gleason: can be ground. The obliquity of the tooth flanks .beta..sub.m may range from 0.degree. to 45.degree.. PA1 b. Klingelnberg: cannot be ground, .beta..sub.m always greater than 0, highly dependent on tooth width imposed by the system, true involute. PA1 c. Oerlikon: cannot be ground, .beta..sub.m freely selectable, true involute.
Despite its simplicity, type 1 is generally out of the question for the prescribed requirements because of its low meshing-quotient. Types 2a, 2b, and 2c all have oblique and curved teeth. Because only the Gleason type can be ground, Gleason toothing is chosen for practically all applications. However, all transmissions of type 2 have the property that the obliquity of the tooth flanks is not constant over the width of the tooth, that is to say that .beta. declines over the width of the tooth. In operation this means that in addition to the constant axial force on the pinion and the gear, resulting from the bevel angle, a continuously varying additional axial force is thus produced in consequence of the variation of the angle .beta. during the engagement of each tooth over its width. This introduces axial vibrations which are already detrimental per se. In the case of serial connection of two transmissions of the types 2a, 2b, or 2c, resonance may therefore easily occur, with all its consequences. For the very light construction required for helicopters, this problem is obviously even more difficult to solve. The consequence is that for many transmissions the required reliability is difficult to achieve, particularly for light-weight constructions. Furthermore, particularly complicated constructions and extremely difficult alignment problems arise, while the displacement of the bevel wheels is in many cases not possible in a straight-forward way. Only heavier types of transmissions appear to be fully adequate. All in all, the gear units in question are among the most difficult in the art and still require considerable improvement.